Seattle is topping one of its famous floating bridges with light rail

The gridlock-relieving rapid transit project is a world's first.

Seattle's trio of floating bridges are already a marvel of modern engineering. By 2023, one of these spans will be reconfigured to include a major rapid transit line. (Rendering: Sound Transit)

As if it weren’t enough for the state of Washington to claim bragging rights as the self-described “floating bridge capital of the world,” transportation officials are beginning preparations to top one of these iconic pontoon-supported spans with a light rail line.

When complete, this huge — in both ambition and innovation — mass transit project will carry Sound Transit's upcoming East Link Extension light rail line across Lake Washington, connecting Seattle to the cities of Bellevue and Redmond along with other well-heeled suburbs located on the lake's eastern shores.

A city wedged in between two large bodies of water, Seattle is home to three of the world’s five longest floating bridges. All of them span Lake Washington, a freshwater ribbon lake that, along with the Puget Sound to the west, gives Seattle its isthmian character.

The Evergreen Point Floating Bridge, which carries State Route 520 over Lake Washington, is the world’s longest at 7,710 feet. Located to the south are the Lacey V. Murrow Memorial Bridge (6,620 feet) and the Homer M. Hadley Memorial Bridge (5,811 feet) — the world’s second and fifth longest floating bridges, respectively. These two bridges are often referred to singularly as the I-90 Floating Bridge as they run directly parallel to each other, carrying traffic eastbound (the Lacey V. Murrow Memorial Bridge) and westbound (the Homer M. Hadley Memorial Bridge) along Interstate 90 from Seattle to Mercer Island. (Connecting the Olympic and Kitsap Peninsulas, the world’s third largest floating bridge, the Hood Canal Bridge, is located two hours northwest of Seattle. The world’s fourth largest floating bridge is about as far away from the Pacific Northwest as you can get … in Georgetown, Guyana.)

It’s the shortest (but also the widest) of Seattle's floating bridges — the Homer M. Hadley Memorial Bridge — that, by 2023, will be home to the world's first-ever floating light rail line. The rail line itself will replace the bridge’s two reversible HOV “express” lanes that carry traffic westbound, toward Seattle, in the morning and eastbound, away from the city, in the evening.

The two I-90 floating bridges carry traffic from Seattle to the Eastside community of Mercer Island. The HOV lanes of Homer M. Hadley Memorial Bridge, pictured in the center, will soon give way to East Link light rail. (Photo: Wikimedia Commons)

Float or bust

For state transportation officials, the decision to do away with Homer M. Hadley Memorial Bridge’s HOV lanes and replace them with train tracks was something of a no-brainer.

For one, building the $3.7 billion East Link to go around Lake Washington was never an option — from a mass transit prospective, circumventing the 22-mile-long lake instead of directly connecting Bellevue with Seattle just didn’t make sense. Carrying the rail line across Lake Washington on a fixed bridge was also a no-go given that the lake is simply too deep to erect columns that could support a conventional bridge. The glacier-carved lake's deepness —110 feet deep on average — is the reason why Lake Washington has floating bridges instead of fixed bridges to begin with. This is also why an underwater tunnel simply wouldn’t work.

While not entirely impossible, constructing a rail-only floating bridge across Lake Washington would have been tricky from an engineering standpoint and also prohibitively expensive.

East Link aims to make traveling between Seattle and the east-of-Lake Washington 'boomburg' of Bellevue — Washington's fourth largest city — less painful for commuters. (Photo: Tiffany Von Arnim/flickr)

“It’s cheaper to do the rail and road bridges together than to separate them,” John Marchione, Redmond mayor and longtime transit board member, recently explained to the Seattle Times.

Issues of lake deepness and cost aside, state transit officials also didn’t have much of a choice to not build the new rail line atop of the Homer M. Hadley Memorial Bridge.

As reported by the Times, in 1976 federal and local governmental leaders signed a pact that would require any third floating bridge built across Lake Washington in the future to include a form of high-capacity transit, be it high-speed bus or rail. That third floating bridge, the Homer M. Hadley Memorial Bridge, was completed 13 years later in 1989. (The original Evergreen State Floating Bridge was built in 1963 and replaced in 2016 while the Lacey V. Murrow Memorial Bridge dates back to 1940 although the original bridge sunk to the bottom of Lake Washington during a freak 1990 storm and was replaced in 1993.)

Although the remarkably wide span was built strong enough to accommodate rail in addition to several lanes of interstate traffic, concerns over load capacity forced the mass-transit aspect to the back burner. Now, after decades of bureaucratic hand wringing, one real estate developer-backed lawsuit and countless rounds of structural testing, that pact made over 40 years ago is finally being honored.

Westbound traffic on the Homer M. Hadley Bridge approaches the Mount Baker Tunnel, a National Register of Historic Places-listed Seattle landmark. The tunnel's HOV lanes will also give way to light rail. (Photo: SounderBruce/flickr)

Applying earthquake science to bobbing bridges

It goes without saying that plopping light rail atop the Homer M. Hadley Memorial Bridge involves a lot more than just squeezing existing HOV lanes into the freeway mainlines of both I-90 bridges. (Kicking off in June, this lane-shifting revamp process alone is a herculean effort with an estimated price tag of $283 million.)

As Sound Transit explains, engineers had to consider six ranges of motion that impact the floating bridge — up and down, back and forth and side to side — while demonstrating that it was absolutely safe to add a pair of 300-ton trains, each moving at up to 55 miles per hour, into the equation.

The Times details the greatest challenge in this no-margin-for-error undertaking:

The most difficult task is adapting the rails to the movements of the bridge. Train tracks will cross the hinges and sloping spans between the bridge’s fixed sections and the 1-mile floating deck, like someone walking down the gangway to a boat marina. Lake levels rise and fall two feet a year. Waves, wind and traffic create slight twisting. A full train is heavy enough to plunge the pontoons eight inches. So the railbed must both resist and absorb roll, pitch and yaw.

Failure is not an option. A derailed train could sink 200 feet to the lake bed. If track components break or wear out, transit service would be halted for maintenance, or subjected to slowdowns.

As John Sleavin, deputy executive director of technical oversight for Sound Transit, explains to local Fox affiliate KCPQ 13: “The bridge goes up and down, also when the wind blows the bridge will go slightly north or south, because it’s on anchor cables much like a boat will kind of move around. And, then as traffic loads, the bridge will also move a little left and right.”

Speaking to the Times, John Stanton, professor of civil engineering at the University of Washington, praises the engineering team's "brilliant solution" that places the railway atop a series of eight 43-foot-long “track bridges” positioned above the hinges where the fixed and floating sections of the bridge meet. Composed of steel plates and high-strength "pivoting" bearings, the technology is the same kind that allows buildings and fixed bridges to flex during earthquakes. With these specialized track bridges, which were relentlessly tested at the Transportation Technology Center in Pueblo, Colorado, trains can cross Lake Washington comfortably at full speed even while the floating bridge deck underneath sways a wee bit to and fro.

What’s more, ballast gravel will be removed from the bridge’s hulking, watertight concrete pontoons to ensure buoyancy and so that the addition of commuter trains don’t throw the bridge off balance.

Due for completion in 2023, Sound Transit's East Link Extension adds 14 miles of light rail line to the traffic-clogged Seattle metro region. Additional extensions are planned or in the works. (Graphic: Sound Transit)

Adds the Times:

In a last-minute design addition, steel frames will be built within the pontoons, so that cables can be pulled through lengthwise. When force is applied at the bridge ends, that should tighten the concrete in the midsections of the pontoons. The goal is to prevent microcracks and assure the 100-year life span of the structure.

Before the trains begin carrying commuters, Sound Transit will run them sans passengers for three months to accurately record track movements. During high winds, train service will be reduced and, in rare cases, temporarily shuttered altogether.

“About once a year we may only allow one train per direction, and about once a decade we may have to cease operations on the bridge until the wind dies down,” Sleavin tells Q13.

Construction of East Link across Lake Washington is not expected to impact the Homer M. Hadley Memorial Bridge’s scenic bike/pedestrian lanes, which are part of the I-90 Mountains to Sound Greenway Trail.

A car-free alternative to a hellish commute

While there’s much more that can be discussed on the technical side (and Times transportation reporter Mike Lindblom does a fantastic job at this), it’s also worth focusing on the impact that connecting Seattle with the Eastside will have on commuters in this congestion-plagued metro area.

Once complete, the 14-mile East Link Extension will ferry commuters from downtown Seattle’s International District/Chinatown to Bellevue, an affluent Eastside satellite city, in just 15 minutes. A ride on East Link from the University of Washington, north of downtown Seattle, to Mercer Island is expected to take 20 minutes. Sound Transit anticipates 50,000 daily riders will hop on East Link for a quick, reliable and headache-free commute — that's a whole lot less cars on the road in a sprawling, historically car-dependent town that recently ranked 10th worst in the nation based on time spent sitting in traffic.

Trains departing from the line's western terminus at International District/Chinatown station — this downtown transit hub is an existing stop on the north-south Central Link line and will serve as a major transfer station — will run parallel with I-90 through the Mount Baker Tunnel, across the Homer M. Hadley Memorial Bridge and beneath Mercer Island’s Aubrey Davis Park, an innovative freeway lid park that covers a portion of the interstate as it passes through the largely residential island. Exiting Mercer Island, trains will then cross the East Channel Bridge, a short fixed bridge that spans Lake Washington’s tech millionaire mansion-lined East Channel. From there, East Link veers away from I-90 and heads north toward downtown Bellevue and the line’s eastern terminus at Overlake, an area just south of downtown Redmond.

Sound Transit's East Link light rail line will connect with Central Link, a north-south line that currently runs 20.4 miles between the city of Seatac and the University of Washington via downtown Seattle. (Photo: Dennis Hamilton/flickr)

The first phase of Sound Transit's East Link Extension will include 11 stations, many with park and ride facilities. Eventually, it will expand even further northward to downtown Redmond.

The 4.3-mile Northgate Link Extension, which expands Central Link from the University of Washington to Seattle's northern patchwork of neighborhoods, is also under construction with an anticipated opening in 2021. In the final planning stages are two additional Central Link extensions, both slated to open in 2023 — the same year that East Link Extension and its game-changing Lake Washington crossing will be up and running. One sees Central Link climb north from north Seattle to the cities of Shoreline and Lynnwood while a southern extension will service commuters in the cities of Kent, Des Moines and Federal Way.

What's more, early this spring Sound Transit announced plans to power its growing light rail system with 100 percent wind energy starting in 2019. Albeit smaller, Sound Transit's wind-powered rail scheme is similar to one that the Dutch government announced in 2015.

“The commute’s getting worse for everybody, I’m seeing it on the 90 for sure,” Brady Wright, a resident of the Eastside city of Issaquah who commutes daily to downtown Seattle for work, tells Q13. “Not being with their families and not being able to do the things you want to do is a big issue, so if you can get an hour back, a half-hour back every day, that’s what people care about.”